DC-DC converters typically craft a DC voltage by full wave rectifying and filtering one or more time varying signals. Because of the switching undertaken in the full wave rectification process, significant amounts of current are frequently “switched” back-and-forth at rapid pace by large transistors. It is often helpful to measure the current through these transistors to, for instance, determine whether or not the DC-DC converter is being loaded, monitor any ripple currents resulting from rectification, etc.
Two “straight-forward” techniques are readily known in the art for measuring current: 1) shunt inductance; and, 2) series resistance. Shunt inductance induces a current measurement signal in an inductor by coupling magnetic fields that are produced by the current signal being measured through the inductor. Unfortunately, shunt inductance is not practical for rapidly changing currents because the bandwidth of an inductor is limited (i.e., the inductor will increasingly attenuate the current measurement signal as its frequency increases).
The series resistance technique, which is shown in FIG. 1, does not typically suffer from limited bandwidth issues because a pure resistance does not change its resistive properties as a function of signal frequency. Unfortunately, however, the series resistance technique is also not practical for large currents (such as those drawn by a DC-DC converter's switching transistors) because a large current being driven through a resistance will tend to dissipate large amounts of power (through the relationship P=I2R) which may result in overheating; or, if the power “problem” is handled by using a very small series resistance, inaccuracy results because the signal V=I*R may become too small to measure.